1. Field of the Invention
The present invention relates to band-pass filters used in radio communication apparatuses and the like for wireless LAN and the like for performing radio transmission between mobile communication apparatuses such as cellular phones and between electronic or electric apparatuses and relates to high-frequency components and communication apparatuses using the band-pass filters.
2. Description of the Related Art
Nowadays, data communication by wireless LAN represented by the IEEE802.11 standard is widely used. The wireless LAN is adopted as signal transmitting means that replaces wired communication between electronic apparatuses such as personal computers (PCs), peripheral equipment of the PC such as printers and hard disks, facsimiles, standard televisions (SDTVs), high definition televisions (HDTV), and cellular phones, in automobiles, and in airplanes. Radio data transmission is performed between respective kinds of electronic apparatuses.
A high-frequency circuit used in a multi-band communication apparatus adopting such a wireless LAN includes one antenna that can perform transmission and reception with two communication systems (IEEE802.11a and IEEE802.11b and/or IEEE802.11g) having different frequency bands and a high-frequency switch that switches connection to a transmission side circuit and a reception side circuit. The high-frequency circuit performs switching of transmission side circuits and reception side circuits of the two communication systems. According to a reduction in size and improvement of functions of radio apparatuses, there is an increasing demand for a reduction in size of high-frequency components, which realize the high-frequency circuit, while integrating a large number of high-frequency components. It is essential to reduce the size of the respective high-frequency components.
Among such high-frequency components, a band-pass filter that selectively causes signal in a predetermined band to pass is an important component of a communication apparatus. The band-pass filter is arranged at a front end of an antenna circuit, between transmission and reception circuits, and the like and used for removing unnecessary waves present near an outside of the pass band.
For example, Japanese Patent Laid-Open No. 6-53704 discloses a laminated band-pass filter. In the band-pass filter disclosed in Japanese Patent Laid-Open No. 6-53704, plural coil electrodes are connected to one another to form spiral electrodes. The spiral electrodes are electromagnetically coupled. A reduction in size of the band-pass filter is realized by such a configuration.
However, in the band-pass filter disclosed in Patent Application Laid-Open No. 6-53704, since the spiral electrodes are still long, loss cannot be sufficiently reduced. Further, in the band-pass filter disclosed in Patent Application Laid-Open No. 6-53704, it is likely that impedance changes because of “misalignment” in a laminating process and a filter characteristic fluctuates. In other words, when coil electrodes formed in respective dielectric layers are misaligned in an in-plane direction, an effective inner diameter of a coil viewed from a projecting direction (a winding axis direction) deviates from a design value and the impedance changes. In this way, the laminated band-pass filter in which electrodes of a resonator are formed over plural layers has a problem of characteristic fluctuation involved in laminating misalignment.